Fix GRUB config creation for PReP boot

Aelinrequested to merge
Aelin/boot-deploy:prep into main

When booting with Open Firmware, no separate partition for /boot is necessary, so the paths for the kernel and initramfs need to be adjusted accordingly.

The deviceinfo_create_prep_boot option is NOT in pmbootstrap yet (pmbootstrap!2704 (merged)), please don't merge this until it is.

Edited by Aelin

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